The present invention pertains to a medical device used to extract foreign objects from a patient. More specifically, the invention relates to an endoscopic device used to retrieve, crush, and remove gallstones and the like. The device is designed to traverse through narrow passages within the body and to open within those passages to retrieve the foreign object.
The removal of foreign bodies from patients often requires the use of endoscopic devices. In particular, gastroenterologists commonly use grasping or crushing devices to extract stones from a patient""s biliary duct. Additionally, snares are often used when removing stents or other foreign objects.
Grasping and crushing devices generally take the form of wire baskets that deploy to capture the stone to be extracted. These wire baskets may be used for lithotripsy if the stone is too large to be removed intact. Lithotripsy involves crushing the stone into fragments to facilitate removal from the duct. Effective performance of such devices requires the baskets to have enough flexibility to be inserted into the common bile duct. However, the baskets also must have a certain degree of rigidity to dilate the duct to facilitate stone capture. Often, the baskets are deployed using a retaining cannula. In this case, the cannula retains the basket in a retracted configuration during insertion into the bile duct. Once within the grasping region of a stone, the basket extends from the cannula and opens to capture the stone. In such a case, the basket must have enough stiffness to open the duct when removed from the cannula, without being so stiff that it is permanently deformed due to retention within the cannula.
Aside from deformation associated with dilating the duct or retention within the cannula, a common failure of conventional baskets occurs during lithotripsy when the baskets are subject to forces often in excess of 50 pounds. Under such force, the basket can become severely deformed, rendering it unsuitable for repeated use. Such repeated use is desirable because of the frequent occurrence of the need to remove more than one stone or other object at a time from the patient. Therefore, design of these devices includes focus on the durability of the basket in repeated use settings.
To repeatedly crush and retrieve foreign objects, a basket must be flexible enough to traverse tortuous anatomy, yet stiff enough to open within a duct, and strong enough to crush stones. A single wire construction may meet any one of these criteria, but typically cannot meet all three requirements for repeated dilation and lithotripsy. It has been proposed, therefore, to construct a retrieval basket of a stranded cable, such as stainless steel cable. Purely stainless steel cable (the core and strands) may work well for the extraction of a single stone, but is subject to the deformation problems discussed previously when used for repeated dilatation or lithotripsy.
Other baskets are formed from cable which includes a superelastic, sometimes referred to as shape memory, core wrapped with strands of stainless steel to surround the core. Nitinol is often used as the superelastic core in these devices. Nitinol is a specially heat-treated Titanium-Nickel (Tixe2x80x94Ni) alloy, preferably approximately 55%/45% Nickel to Titanium (Nixe2x80x94Ti). These baskets require heat treatment for the core to retain its shape. Such a configuration allows for some improvement in performance when the baskets are used repeatedly and for lithotripsy because the superelastic core better retains its shape.
However, superelastic materials of this type experience phase transformations when subject to a certain level of stress loading. Lithotripsy often reaches these stress levels. Upon a phase transformation, the core of the cable stretches, rendering the device incapable of transferring force to the stone to complete the crushing process. Furthermore, the superelastic alloy has a greater reversible elongation than do the surrounding stainless steel strands. This results in a difference in deformation between the core and the surrounding strands leading to a permanent deformation of the cable. Such deformation results in an alteration of the basket shape, making it less desirable to use for its intended purpose.
Moreover, manufacturing both the cable core and strands from superelastic alloy wires results in a cable that unwinds due to the highly elastic nature of the material. Thus, a retrieval basket of such cable also will not retain its desired shape without heat treating.
The advantages and purpose of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages and purpose of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.
To attain the advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention includes a medical retrieval device for retrieving foreign objects from within a patient""s body. The retrieval device includes a retrieval assembly containing a cable preformed into a configuration for capturing and removing the foreign object. The retrieval cable includes wire made of a precursor alloy to a superelastic material. According to a particularly preferred embodiment of the invention, the cable includes a core wire and surrounding wire strands, each made of the precursor alloy.
The invention further includes a method of manufacturing the medical retrieval device including the steps of constructing a cable including a wire made of a precursor alloy to a superelastic material and forming a retrieval assembly by preforming the cable into a configuration adapted to capture and remove the foreign objects.
The precursor alloy according to the present invention exhibits a stress-strain curve having a linear relationship extending through a yield point with no phase transformation point. After the yield point, the stress-strain curve does not exhibit a substantially constant stress plateau as strain increases. Rather, the precursor alloy exhibits plastic deformation properties.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.